Especially for heavy road vehicles, such as trucks and buses, so called S-cam drum brakes are often used. Within the brake drum brake shoes provided with brake linings may be pressed apart by means of a generally S-shaped cam rotatable with a shaft (called the S-cam shaft) extending out of the brake drum. A lever, called the brake lever, attached to the S-cam shaft is connected to a push rod of a preferably pneumatic brake cylinder in the vehicle underframe. Thus, at the admission of air under pressure to the brake cylinder a brake force will be transmitted from said push rod via the brake lever to the S-cam shaft and the S-cam, which will press apart the brake shoes and thus apply the brakes.
When wear of the brake linings occurs, a longer stroke of the brake cylinder push rod will be necessary before the brake is applied. Earlier it was customary to manually compensate for this wear by adjusting the angular position of the brake lever relative to the S-cam shaft, preferably by manually turning a shaft of a worm screw in engagement with a worm wheel, which is attached to the S-cam shaft.
Later it has become common to provide the brake lever with internal means for automatically turning the worm screw shaft and thus adjusting the angular position of the brake lever in dependence on the brake lining wear; a so equipped lever is in the art called an automatic brake lever.
Most of the designs for such automatic brake levers are based on the so called piston stroke principle, i.e. the adjustment depends entirely on the piston rod or push rod stroke or in other words the angular movement of the brake lever past a certain value corresponding to the normal clearance between the brake linings and the brake drum in the rest position.
More recently it has been found that different reasons speak for a more advanced principle--the so called clearance sensing principle. In this case the adjusting mechanism is able to differentiate between the push rod stroke depending on the wear of the brake linings and that depending on the often considerable elasticity in the different parts between the brake cylinder and the brake drum. This means that the automatic adjustment reduces the clearance to the normal and desired value when it has become excessive, due for example to wear of the brake linings, whereas the mechanism ignores the influence of the elasticity.
Such clearance sensing automatic brake levers or slack adjusters are for example known through U.S. Pat. No. Re. 26 965, U.S. Pat. Nos. 3,507,369, 3,901,537, 4,114,733, 4,121,703, 3,997,035, 3,997,036, 4,015,692 and 4,019,612.
Different requirements are imposed on a product of this kind. Generally speaking an automatic brake lever is a safety device working under extremely hard conditions as regards loads and environment. Further, the available space for the brake lever is often very limited. The reliability must be high and the periods between normal services as long as possible. Last but not least the price must be competitive.
Still further it is desirable to obviate the necessity for mounting the brake lever in a certain predetermined position, which is inconvenient not only at the initial mounting but even more after later servicing. The automatic brake lever shall thus preferably have a so called floating reference point or fixpoint.